[Build] use snapshots of debian mirrors for sonic-slave containers #14400

Why I did it
We don't use snapshots of debian mirrors for sonic-slave containers even if MIRROR_SNAPSHOT is enabled.

How I did it
Export MIRROR_SNAPSHOT in Makefile.work to generate sources.list for sonic-slave containers using debian snapshot mirror

How to verify it

Makefile.work

@@ -581,6 +581,7 @@ endif
 
 export MIRROR_URLS
 export MIRROR_SECURITY_URLS
+export MIRROR_SNAPSHOT
 export SONIC_VERSION_CONTROL_COMPONENTS
 
 %:: | sonic-build-hooks